Many advances have been made recently in the field of printing. Great demands have been made upon printing systems and materials therefor in light of the need of faster printing, in conjunction with the computer age, and the seeking of lower toxicity to comply with governmental environmental regulations. In high speed printing, a problem has been encountered with paper dust which is generated during paper transport and the impact of a printing element on the paper. This is particularly true of printing on envelopes which are made of paper with loose fibers.
One of the components of the printing system that has required extensive modification is the ink. In most printing applications, a pad or roller made of a porous material is supplied with ink for the purpose of transferring the ink to a printhead. The printing pad may be one which is self contained or one that is periodically supplied with ink from a reservoir.
Problems in producing a satisfactory ink are compounded when one seeks to produce an ink that results in printing wherein the printed matter can be seen visually in ordinary light and can be seen having the same or a different color when exposed to ultraviolet light. These inks are known as fluorescent inks and are useful because of their ability to be detected by an optical device. Such inks have wide spread use in the mail handling field where devices, referred to as postage meters or franking machines, print an indicia upon mail pieces. The fluorescent ink yield the ability to determine the location of an indicia on a mail piece and discern the authenticity of postage.
The problems in producing a satisfactory fluorescent ink are compounded because a plurality of dyes and pigments are required for achieving good color, fill in, edge acuity and the other qualities required from an ink. One of the major problems associated with inks is aging which results in printing that has a change in color and intensity with the passing of the time the ink is stored. An ink that overcomes the aging problem and shows low environmental sensitivity at high humidity and low and high temperature, low tackiness and flow properties, material compatibility and paper dust resistance would be advantageous. Another consideration is that the ink pad must function in an open environment without drying for at least six months; therefore, the ink solvents should be non volatile.